The present invention relates to projection systems such as slide film projection systems and motion picture projection systems, and more particularly to the lighting system utilized in such projection systems. In such projectors a rectangular aperture of moderate size is desired to be illuminated with a light beam so that the light crossing the aperture has a fairly even distribution over the entire screen.
Projection systems, such as slide film projectors, opaque projectors and motion film projectors, in general, are composed of a light source, optical elements that collect the light and project it as a light beam onto an aperture, and additional optical elements that project an image of the aperture onto a viewing screen. When a film transparency, such as a motion film picture film, is introduced into the aperture, the image depicted on the film is projected onto a screen.
It has been considered generally desirable to achieve as even a distribution of illumination on the screen as is attainable and to seek a high efficiency in the transfer of light from the light source to the screen. High efficiency in light transfer is desirable because it means that less power is required from the lamp, which permits the use of a smaller lamp and generates less heat at the film aperture (also called the "gate"). These advantages of high light transfer efficiency tend to reduce the construction and operating cost of projection systems. For example, if the light source is an xenon arc lamp of 600 watts instead of an xenon arc lamp of 1,000 watts, the size and the cost of the power supply may be considerably reduced, the size and cost of the reflector and casing may be reduced, and a less expensive lamp may be utilized.
At the present time projection systems, such as slide film projection systems and motion picture projection systems, utilize a variety of internal light sources, depending upon such factors as the desired degree of even light distribution on the screen, the desired efficiency of light transfer from the light source to the screen, the level of illumination, the cost of the system, and the frequency of its use. Generally speaking, if it is desired to have a brighter picture, for example, permitting the image on the screen to be viewed under daylight conditions, then the illumination source must provide a relatively large quantity of light. On the other hand, slide projectors for viewing at home may use a relatively low level illumination source. For example, the Carousel (trademark of Kodak) line of slide film projectors uses a 500-watt tungsten lamp or a 500-watt quartz halogen lamp having a tungsten filament as its illumination source.
In those projectors in which it is desired to project a color slide under daylight conditions on a relatively large screen, for example, 20 feet by 20 feet, it is considered necessary to use a relatively high-powered xenon bulb as the light source. The brightest projection systems presently commercially available utilize xenon arc lamps as their light source. Xenon arc lamps are arc lamps having a gas-tight bulb, usually of quartz and filled with xenon gas. A xenon arc lamp will exhibit very high average luminance over its light emitting area, i.e., over the arc plasma, many times higher luminance than commonly used incandescent projection bulbs. For this reason xenon arc lamps are admirably suited for projector applications. A conventional daylight slide projector for projection upon a 20 .times. 20 ft. screen may utilize an xenon arc lamp of about 1,000 watts power.
Generally, in high quality projection systems using xenon arc lamps, the arc lamp is positioned in front of, and at the center of, a reflector. Xenon arc lamps are presently conventionally mounted on deep-dish elliptical reflectors. Such reflectors focus the emitting area of the arc, situated normally at the primary focus of the ellipse, onto an aperture (gate) which is situated at the secondary focal point of the ellipse. Such a lamp/reflector combination produces a circular pattern of light at the aperture; however, the aperture is normally rectangular in slide and film projection systems. In order to fill the aperture properly with relatively even illumination, the arc point of the arc lamp has to be moved relative to the primary focus of the elliptical reflector, in effect diffusing the arc image obtained at the aperture. This adjustment achieves a more even distribution of light across the aperture but decreases the efficiency of light transfer in that it causes substantial losses in the total amount of light that ends up on the projection screen.
Although the deep-dish elliptical reflector is almost universally used in commercial apparatus, other types of reflectors have been suggested from time to time in the past. The U.S. Pat. No. 1,248,456 to P. L. Clark, patented Dec. 4, 1917, shows at FIGS. 12 through 14 a four-part reflector having "independently adjustable sections." Each section of the reflector appears to have a curvature, although the specific curvature is not described. The patent states, in respect to its FIG. 2, that the plurality of concave mirrors "may be ellipsoidal, spherical, parabolic or of other curvatures approximating the above, or a combination of two or more of them" (page 1, lines 61-64). The sectional reflectors of the Clark patent, consequently, appear to be sections of an ellipsoid sphere or a parbola, each of the sections having a common axis through the imaginary point where the points of the sections would meet.